Only source varstash lib if it's being used

Only source varstash lib if it's being used

I have noticed that `autostash` called an (accidentally)
defined/overwritten `stash` function.

While it's not possible to put this into a local scope, this will at
least make it less polluting by default (when autostashing is not used).

Additionally we could source it always (not just once), but then it
would only re-overwrite the function.

A better fix would be to prefix the functions explicitly, e.g.
`autoenv_autostash` etc.

Closes https://github.com/Tarrasch/zsh-autoenv/pull/42.